1. Field of the Invention
This invention relates to a method of preparing polyborate esters and the use of these esters in lubricants. More particularly, it relates to a single step process for the preparation of polyborate esters from ortho-boric acid which are useful as wear and oxidation inhibitors in lubricants.
2. Description of the Prior Art
Polyborate esters can be regarded as partially esterified boron oxides. More specifically, these materials contain at least two boron atoms which are attached to each other through an oxygen atom bridge. In addition, they also contain at least one borate ester group of the formula R--O--B&lt; wherein R is either a substituted or unsubstituted hydrocarbyl group. These materials can be linear, branched or cyclic in character. Included within this class are the bis-borate esters such as (RO).sub.2 B--O--B(OR).sub.2, (RO)(HO)B--O--B(OR).sub.2, (HO).sub.2 B--O--B(OR).sub.2, and (HO).sub.2 B--O--B(OH)(OR); metaborate esters of the formula: ##STR1## and also a plethora of more complex polymeric materials which, frequently, have a structure which is unknown in detail.
It is known that boric acid can be esterified by reaction with an alcohol as set forth in the following equation: EQU 3ROH+H.sub.3 BO.sub.3 .revreaction.(RO).sub.3 B+3H.sub.2 O
It has also been disclosed by U.S. Pat. No. 4,303,445, issued to Whitfield et al. on Dec. 1, 1981, that bis-borate esters can be prepared by the direct reaction of boric acid with an alcohol. However, the borate and polyborate esters are hydrolytically unstable, and water is a product of this type of reaction. As a consequence, this method of preparation is usually not satifactory unless the water can be removed and the reaction forced to completion. The removal of this water is a difficult problem which has not been fully solved.
It is also known that polyborate esters can be prepared by the reaction of boron oxide with esters of ortho-boric acid. However, this method is frequently impractical because of a relatively slow rate of reaction. Examples of this approach are set forth in U.S. Pat. No. 3,099,677, issued to Hunter on July 30, 1963, and U.S. Pat. No. 3,522,286, issued to Salvemini on July 28, 1970.
U.S. Pat. No. 3,755,408, issued to Cuneo on Aug. 28, 1973, discloses a two-step process for the preparation of polyborate esters wherein the first step involves heating a suspension of ortho-boric acid (H.sub.3 BO.sub.3) at a temperature between 80.degree. and 220.degree. C. in an inert organic liquid to dehydrate the acid, and the second step involves reacting the resulting material with an alcohol or phenol. This patent does not, however, either teach or suggest that these two steps could be combined. Similarly, U.S. Pat. No. 3,202,492, issued to Liao on Aug. 24, 1965, teaches that metaborate esters can be prepared by mixing a monohydroxy alcohol and boron oxide in a 2:1 molar ratio together with an organic solvent, such as benzene, and heating at reflux to azeotropically remove the water of reaction.
U.S. Pat. Nos. 2,721,121 and 2,721,180, both issued to Lawrence et al. on Oct. 18, 1955, disclose that oil-soluble organo-boron compounds can be prepared by passing an oxygen-containing gas through a liquid petroleum fraction having a boiling point in the range from about 350.degree. to about 800.degree. F. (177.degree. to 427.degree. C.) and in the presence of an acid selected from the group consisting of ortho-boric acid and meta-boric acid. It is stated that the products of this process can be described, at least in part, as mono-borate esters and bis-borate esters. It is further disclosed that these products are useful as additives to improve the oxidation or wear characteristics of lubricant compositions.